The physical properties which are important in engine poppet valve applications include high temperature creep and fatigue strengths in the head which is the portion of the valve that is subjected to the high operating temperatures of the combustion chamber, and good low temperature fatigue and tensile strengths in the stem near the keeper groove.
In making valves from the many austenitic alloys that are available, it is a conventional practice to solution heat treat the valves in a batch process. The conventional solution heat treatment process has several disadvantages. When the time and temperature are selected to achieve a microstructure having a large grain size for optimum high temperature properties in the head, there is a sacrifice of low temperature properties in the stem. Conversely, when the time and temperature of heat treatment are selected to achieve good low temperature properties in the stem, it is not possible to obtain the best high temperature properties in the head. Batch-type solution treatment processes tend to cause distortion of the valve stems which makes it necessary to employ a roll straightening operation. Another disadvantage is that it is usually necessary to completely age the valves after solution treatment in order to avoid strain-age cracking associated with roll straightening of the stems. Still other disadvantages of the conventional batch-type solution heat treatment process include the need for an endothermic atmosphere, the processing time that is required, and a general inability to achieve a consistent microstructure from valve-to-valve.